1. Field of the Invention
The present invention relates to an optical waveguide device and traveling wave type optical modulator.
2. Related Art Statement
A traveling-wave optical modulator using lithium niobate. (LiNbO3), lithium tantalate (LiTaO3) or gallium-arsenide (GaAs) for the optical waveguide has excellent properties and may realize a broadband modulation at a high efficiency. Lithium niobate and lithium tantalate are excellent ferroelectric materials having large electrooptic coefficients and can control light within a short optical path. Factors suppressing the modulation frequency of the traveling-wave optical modulator include velocity mismatch, dispersion, electrode conductor loss and dielectric loss.
The concept of velocity mismatch will now be further explained. In a traveling-wave optical modulator, the velocity of the light propagating through the optical waveguide largely differs from that of the signal microwave propagating through this electrode. Assume that the light and the modulation wave propagating through the crystal have different velocity Vo and Vm, respectively. For example calculation was made for an LiNbO3 optical modulator having planar type electrodes. The effective refractive index of LiNbO3 single crystal is 2.15, and the velocity of the light propagating through the optical waveguide is inversely proportional to the refractive index. On the other hand, the effective index for modulating wave is given by a square root of the dielectric constant near a conductor propagating the wave. LiNbO3 is an uniaxial crystal, and the dielectric constant in the Z-axis direction is 28 and those in the X-axis and Y-axis directions are 43. Therefore, even if an influence of air having a dielectric constant of 1, the effective microwave index in the LiNbO2 optical modulator having a conventional structure is about 4, which is about 1.9 times larger than 2.15. Thus, the velocity of the light wave is about 1.9 times as much as that of the modulating microwave.
The upper limit of bandwidth “fm” of the optical modulator or the modulating velocity is inversely proportional to a difference in velocity between the lightwave and the modulating microwave. That is, fm=1/(Vo·Vm). Therefore, assuming that the power loss by electrode is zero, the upper limit of bandwidth “fm”×electrode length “1”=9.2 GHz.cm. Actually, it has been reported that in an optical modulator having an electrode length “1”=2.5 mm, fm=40 GHz. The effect due to the upper limit of modulation speed becomes more substantial as the electrodes become longer. Therefore, an optical modulator having a broadband modulation and low driving voltage has been earnestly demanded.
The inventors have considered the following idea. That is, the velocity matching between signal microwave and lightwave may be realized by applying a thin plate with a thickness of, for example, 10 μm for an optical waveguide substrate.
The assignee filed a Japanese patent publication 10-133, 159A (6,219,469), and disclosed a travelling wave optical modulator for giving the solution. The modulator has an optical waveguide substrate having a thinner portion with a thickness of not more than 10 μm where the optical waveguide is formed. It is thereby possible to realize high-speed modulation without forming a buffer layer made of silicon dioxide, and to advantageously reduce the product “Vπ·L” of a driving voltage Vπ and a length of an electrode “L”.